1. Field of the Invention
The present invention relates to a luminance signal processing device such as an image signal processing device, e.g., capable of attaining a high-definition image by obtaining a wideband digital luminance signal from an image signal output from an image sensor developed for use in digital cameras.
2. Description of the Related Art
In the camera industry, there has been a remarkable shift from analog to digital technology in recent years. In particular, digital still cameras, which do not require any films and development, are selling very well, and as for mobile phones, those with a camera are the mainstream.
At present, in the field of digital cameras, for example, the mainstream cameras adopt a sensor that places emphasis on color reproducibility and is thus equipped with a primary color filter, and signal processing associated with the sensor. Such a camera needs digital signal processing that places importance on resolution and provides excellent color S/N. In view of this, an imaging device and the like, in which noise can be removed while resolution decrease is suppressed, have been proposed (see, e.g., Japanese Laid-Open Publication No. 2001-189944).
This type of imaging device includes a luminance signal processing device which is configured as shown in FIG. 9, for example. In FIG. 9, the reference numeral 501 denotes a preprocessing section for performing black level correction, gamma correction, gain correction and the like for image signal data obtained by subjecting the output of an image sensor having an RGB (red, green and blue) Bayer pattern filter to noise removal process, amplification process, and A/D conversion process. The reference numeral 502 represents a multiple line output converter, which converts the output of the preprocessing section to parallel outputs in which data sets in a plurality of horizontal lines are output in parallel. The reference numeral 503 indicates a low-pass filter, which removes color carrier components, occurring due to the Bayer pattern of the sensor and superposed on the output of the sensor, so as to generate luminance signal data. The reference numeral 504 denotes an edge enhancement correction section, which performs edge enhancement of the luminance signal data by extracting the high-frequency components from the luminance signal data obtained after the color carrier component removal and adding the extracted components to the original luminance signal data. The reference numeral 505 indicates a postprocessing section for adjusting the bias level of the luminance signal data, the gain and the like.
FIG. 10 indicates frequency components of the output of the above-described sensor.
In the luminance signal processing device described above, as the first step of the processing, the low-pass filter processing is performed so that the high-frequency color carrier components in the vicinity of the Nyquist frequency are removed to generate the main signals of the luminance signals. By using these signals, the edge enhancement correction is carried out. In this device, the luminance signal processing is performed frame by frame and the characteristics of the low-pass filter 503 for removing the color carrier components are uniform in all portions within the single image area.
Nevertheless, the conventional luminance signal processing device described above has a problem because of the fact that the characteristics of the color-carrier-component-removing low-pass filter 503 are uniform in all portions in the single image area; if the pass-band of the low-pass filter 503 is increased toward the high frequency side in order to increase the resolution in a specific color portion, e.g., a skin color portion, a false edge signal is produced from the remaining color carrier components at a boundary where the colors change greatly in the RGB Bayer pattern, which results in inappropriate edge enhancement.